Celaena isn't just tough. She's complicated. She kills people and then cries about her dog. She's vain about her appearance while covered in blood. That specific combination of lethal competence and emotional messiness is rare, and these books get closest to it.
An Ember in the Ashes by Sabaa Tahir puts you in a military empire where Laia is a slave and Elias is a soldier who doesn't want to fight. The brutality here is real. Tahir doesn't shy away from what oppression looks like, and Laia's growth from terrified girl to someone willing to risk everything mirrors Celaena's arc across the TOG series. The romance builds under pressure, which is the only way it works in a setting this violent. Four books, all of them escalating, all of them worth it.
The Poppy War by R.F. Kuang is darker than TOG. Significantly darker. Rin is a war orphan who tests into a military academy, discovers she has shamanic powers, and then watches her country get invaded. This is inspired by Chinese history and it doesn't pull punches. If you loved the parts of TOG where Celaena stopped being charming and became something terrifying, Kuang writes that transformation better than almost anyone.
Graceling by Kristin Cashore features Katsa, who has been used as a weapon since childhood. Her Grace (a supernatural talent) is killing, and the story follows her figuring out that she's more than what she was made to be. The romance with Po is one of the best in YA fantasy because it's genuinely equal. Neither character needs the other to survive. They choose each other, which is always more interesting. And it's a standalone, so you get the full arc in one book.
Fourth Wing by Rebecca Yarros puts Violet in a dragon-rider war college where students die regularly. She's physically smaller than everyone else, which means she has to be smarter and more stubborn. Sound familiar? The Celaena energy is strong here. The enemies-to-lovers romance with Xaden is intense, the dragons have actual personalities, and the found family that forms under pressure is one of the best in recent romantasy.
Spark of the Everflame by Penn Cole gives you Diem, a healer forced into a world of immortal politics where her very existence threatens the power structure. She's got that same Celaena stubbornness, the refusal to stay quiet when smart people would shut up. The series just got picked up by Hulu, and Book 4 drops in September. Perfect time to start.